This invention relates generally to nanostructured steel and a method of making the same. More particularly, this invention relates to nanostructured M50 type steel synthesized by chemical methods, which has improved mechanical and physical properties such as hardness, strength and durability.
Metals and alloys are traditionally produced by melting and casting techniques which entail some level of microstructural and chemical inhomogeneities. Recent developments in rapid solidification techniques, such as powder atomization and melt spinning, are capable of producing chemically homogeneous materials with fine microstructures. Grain sizes in micrometers are achievable by rapid solidification techniques. The microstructural refinement is usually accompanied by enhanced mechanical and physical properties. In recent years, much attention has been devoted to a further reduction of grain size from the micrometer size to the nanometer. Nanostructured materials have superior mechanical, magnetic, and other physical properties.
Iron-based alloys are technologically important materials in modern industry. For example, M50 steel (4.0%Cr, 4.5%Mo, 1.0%V, 0.8%C, with balance of Fe), because of its good resistance to tempering, wear and rolling contact fatigue, has been used extensively in the aircraft industry as main-shaft bearings in gas-turbine engines. In the hardened condition, M50 steel consists of a body-centered tetragonal martensite phase and a dispersion of carbide particles including M.sub.23 C.sub.6 M.sub.6 C, M.sub.2 C and MC. The grain size of the martensite is about 0.032 mm and smaller, and some of the dispersion particles are several microns in diameter. These relatively large carbide particles often act as fatigue crack initiation sites. A nanostructured M50 steel would not contain these large carbide particles. Furthermore, a nanostructured M50 steel may have improved resistance to tempering, and wear and rolling contact fatigue.
Techniques for the production of nanostructured materials include electrolytic deposition, chemical or vapor deposition, mechanical alloying and chemical synthesis. Chemical synthesis is advantageous in that it allows tailored synthesis through assembly of atomic or molecular precursors, allows control of stoichiometry, allows mixing of constituent phases at the molecular level, and provides for faster, cost-effective production of bulk quantities of materials.
Examples of the chemical synthesis of ultrafine iron and iron-cobalt alloys have been described in U.S. Pat. No. 4,842,641 issued to Gonsalves; by Jaques van Wonterghem et al in an article entitled "Formation of a Metallic Glass by Thermal Decomposition of Fe(CO).sub.5 " in Physical Review Letters, Vol. 55, No. 4, pages 410-413 (1985); by Jaques van Wonterghem et al in an article entitled "Formation of Ultrafine Amorphous Alloy Particles by Reduction in Aqueous Solution" in Nature, Vol. 322, pages 622-623 (1986); and by Kenneth E. Gonsalves and Kuttaripalyam T. Kembaiyan in an article entitled "Synthesis of Advanced Ceramics and Intermetallics From Organometallic/Polymeric Precursors" in Solid State Ionics , 32/33, pages 661-668 (1989). However, there are no previous reports of the chemical synthesis of a multicomponent commercial nanostmctured M50 steel.